I asked about that in the comments on that page but got no reply. I think I also posted a question about it before on here. Anyway, I think days with 9 in, especially the 29th are good days for chod for the same reason they are good for protector offerings/practice and obstacle dispelling practice. But then, I'm not sure what that reason is.

I don't know either, but I do know that on these days I'm more prone to the kleshas of anger and associated emotions. I can find phenomena more prickly on these days. I think there is something about that energy associated with these practices on these days. But you'd have to ask a knowledgeable lama.

"When a Dzogchen Yogi hears Shakyamuni Buddha turning the Wheel of the Dharma of the Four Noble Truths he hears Samathabhadra proclaiming the most profound Dzogpachenpo." - Nyoshul Khen Rinpoche